Magnetic Resonance Imaging (MRI) is one of the most important modern medical imaging modalities. It has far less risk of side effects than most other imaging modalities such as radioscopy with x-rays or computed tomography because patients and medical personnel are not subjected to ionizing radiation exposure in the procedure. The use of MRI has grown very fast. Every year, more than 30 million MRI scans are performed in the United States; more than 60 million MRI scans are performed worldwide. Doctors often recommend MRI for the diagnosis of various diseases, such as tumors, strokes, heart problems, and spine diseases. A high-quality scan is important for maximizing diagnostic sensitivity and accuracy. Generally, high quality images are characterized by high signal to noise ratio (SNR), high contrast between normal and pathological tissues, low levels of artifacts, and appropriate spatial-temporal resolution.
In order to obtain a detectable MR signal, the object/subject examined is positioned in a homogeneous static magnetic field so that the object's nuclear spins generate net magnetization oriented along the static magnetic field. The net magnetization is rotated away from the static magnetic field using a radio frequency (RF) excitation field with the same frequency as the Larmor frequency of the nucleus. The angle of rotation is determined by the field strength of the RF excitation pulse and its duration. In the end of the RF excitation pulse, the nuclei, in relaxing to their normal spin conditions, generate a decaying signal (the “MR signal”) at the same radio frequency as the RF excitation. The MR signal is picked up by a receive coil, amplified and processed. The acquired measurements, which are collected in the spatial frequency domain, are digitized and stored as complex numerical values in a “k-space” matrix. An associated MR image can be reconstructed from the k-space data, for example, by an inverse 2D or 3D fast Fourier transformation (FFT) from the raw k-space data.